Catalysts containing vanadium and phosphorus oxides have been used in the oxidation of 4-carbon atom hydrocarbons, such as n-butane, n-butenes, 1,3 butadiene or mixtures thereof with molecular oxygen or oxygen-containing gas to produce maleic anhydride.
Representative vanadium-phosphorus catalysts and methods of their preparation are disclosed in U.S. Pat. Nos. 4,043,943 and 4,132,670 and vanadium-phosphorus-uranium catalysts and methods of their preparation are disclosed in U.S. Pat. Nos. 4,002,650 and 4,172,084, the latter two patents being assigned to our common Assignee of record herein. These catalysts may be utilized in either fluid bed or fixed bed reactors for the production of maleic anhydride. Before the catalytic material can be utilized, however, it must be converted from its post-preparation form, usually a dry powder, to a form suitable to be charged to a commerical reactor.
A catalytic powder is conventionally compressed into the form of pellets or tablets for use in commercial reactors. This procedure is costly, time consuming, and rather difficult as it requires subjecting the powder to high pressures, exemplified by European Patent Application No. 3,431 as being on the order of 16 tons per sq. in. to 27 tons per sq. in.
When pellets are utilized, the catalytic material on the interior of the pellet often does not participate in the reaction, because the reactant often cannot diffuse far enough into the interior of the pellet for the catalytic material which is present there to affect the reactants. Because this high cost catalytic material does not participate in the reaction, it is essentially wasted.
One alternate method of forming catalytic material so that it can be utilized in a commercial reactor is the coating of the catalytic material on a support or carrier, as described in U.S. Pat. No. 4,077,912, owned by our common Assignee. A coating method was utilized with catalysts useful in preparing maleic anhydride from benzene as described in U.S. Pat. No. 4,097,501, also owned by our common assignee. These patents disclose that the reactions thus catalyzed are most favorably effected when the amount of the active catalyst material is present as a coating on a support or carrier in the amount of about 10-100% by weight of the support, that is, in the amount of about 5-50% of the combined weight of catalytic oxide material and support.
Catalysts for the oxidation of 4-carbon atom hydrocarbons such as butane do not, however, give acceptable yields when present as a coating in these proportions. When the catalytic oxide material is present in an amount less than 50% of the combined weight of the catalytic oxide material and support, the yield of the maleic anhydride from 4-carbon atom hydrocarbon feedstock is generally lower than would be achieved if the catalyst were in pelleted form.
U.S. Pat. No. 4,071,539 discloses catalysts consisting of oxides of vanadium, phosphorus, copper, and cerium, and oxides of vanadium, phosphorus, copper, tellurium, cerium and an alkali metal or an alkaline earth, useful in the preparation of maleic anhydride from 4-carbon hydrocarbons, which catalysts may be coated as a catalyst precursor solution, or as a catalyst precursor paste onto carriers and thereafter dried and calcined. In order to achieve yields of maleic anhydride with coated catalysts comparable to yields achieved with non-coated catalysts, the amount of catalytic material in these coatings was required to be in the range of about 85 to 92.5% of the combined weight of the carrier and catalytic material. The advantage of partially filling the reactor space with low cost carrier rather than high cost active material, while achieving the same or better yield, is lost when the amount of carrier material used is relatively insignificant, that is, when the amount of carrier is less than about 20% of the combined weight of the carrier and catalytic material.